Orthopaedic devices are used in many situations to stabilize and/or support bones and other tissues, such as fractured bones and bone fragments. During attachment of an orthopaedic device, it is sometimes necessary or beneficial to target one or more landmarks of the orthopaedic device. For example, obscured landmarks, such as fixation holes of an implanted orthopaedic device can be located using jigs and/or x-ray imaging. Once located, a tool and or a fastener can be engaged with the landmark. Additionally, visible landmarks can also be targeted to ensure proper or desired alignment of a tool or fastener with a landmark of the orthopaedic device. For example, a desired angle of insertion of a tool or a fastener relative to a landmark of the orthopaedic device can be achieved using a mechanical jig.
Alternatively, landmarks of orthopaedic devices can be targeted using electromagnetic spatial measurement systems, which determine the location of orthopaedic devices that are associated with inductive electromagnetic sensors in the form of sensor coils. When the orthopaedic device is placed within a magnetic field, voltage or current is induced in the sensor coils, which can be used by a measurement system to determine a position of the orthopaedic device. As the magnetic fields are of a low strength and can safely pass through human tissue, position measurement of the orthopaedic device is possible without line-of-sight constraints of optical spatial measurement systems.
In mechanics, degrees-of-freedom (DOF) are the set of independent displacements and/or rotations that specify a displaced location and rotational orientation of an object. For example, a particle that moves in three dimensional space has three translational displacement components and therefore three degrees-of-freedom (3 DOF). Translation is the ability to move without rotating, while rotation is angular motion about some axis. In contrast to a particle, a rigid body would have a maximum 6 DOF including three rotations and three translations. Specifically, in the case of a rigid body with d dimensions, the body has d(d+1)/2 degrees of freedom (d translations and d(d−1)/2 rotations). Therefore, a rigid body with three dimensions (X, Y and Z), has a maximum 6 DOF. As used herein, a position of an object includes translational locations and rotational orientations that define the position of the object in three dimensional space.
Currently available electromagnetic sensors may be embedded in or attached to an object such as a surgical instrument or orthopaedic implant to allow the position of the object to be displayed in the correct anatomical context in real-time. Referring to FIGS. 1-2, electromagnetic sensors 10 may be arranged in a cylindrical body or rod 11. The sensors 10 comprise two coils 12, 13 placed on top of each other in a crosswise configuration with a printed circuit board (PCB) 14 disposed between the coils 12, 13 to protect the coils 12, 13 from breakage. If only a 5DOF sensor is needed for a spatial tracking application, the arrangement illustrated in FIGS. 1-2 is acceptable because the rotation about the diameter of the cylinder or the z-axis (FIG. 2) may be disregarded. However, the arrangement of FIGS. 1 and 2 requires the structure that houses the sensor 10 to be at least twice as thick as the diameter of the coils 12, 13 to accommodate the crossing configuration of the coils 12, 13 and the body or rod 11. Further, because 6 DOF sensors are required or beneficial for some targeting applications, e.g., targeting a distal end of an orthopaedic implant, such as an intramedullary nail, the circular cross-sectional shape of the sensor housing 11 (FIG. 1) makes it difficult to consistently place the sensor 10 in a predetermined orientation in the implant because the cylinder 11 tends to rotate about its z-axis (FIG. 2) during assembly and, as a result, the predetermined orientation of the coordinates may change during assembly.